I leave my desktop computer on all the time 24/7 unless there is a thunderstorm that could strike and ruin the computer then I shut the computer off and unplug it just to be safe from the lightning strike.
But other than that my desktop computer never gets shut off and I always download the large files overnight or when I'm away from the house and don't need to use the computer for anything else.
Web servers are left on 24/7 such as for this website.
The web server never shuts down otherwise you couldn't connect to the website.
So leaving your desktop computer on overnight or all the time is okay as well and nothing bad will happen to it.
It's actually better to leave a desktop computer on all the time instead of shutting it down because it causes less wear and tear on it.